Method for selecting an insert on the basis of the mechanical resistance thereof

ABSTRACT

Method for selecting a polymeric insert intended to be chosen for its mechanical strength qualities so as to be incorporated into the construction of laminated glazing, the method consisting in evaluating the tear strength of the insert, characterized in that it also consists in evaluating the adhesion of said insert to at least one glass substrate

[0001] The invention relates to a method of selecting a polymeric insertthat has to be chosen for its mechanical strength qualities and isintended, for example, to be used in laminated glazing, preferablygiving the glazing acoustic properties.

[0002] The term “polymeric insert” is understood to mean a monolithicinsert or a composite insert consisting of the assembly of severalpolymeric elements in the form of layers, resins or films. Preferably,at least one of the elements incorporates polyvinyl butyral (PVB).

[0003] Laminated glazing is generally intended for fitting into vehiclesor buildings. It has major assets from the standpoint of its mechanicalstrength. This is because, when an impact occurs, before the glassfractures, the insert advantageously allows some of the energy to beabsorbed by viscous dissipation. The role of the insert is alsoparamount since it guarantees to a great extent the integrity of thestructure when the glass is completely shattered, making it possible,thanks to the adhesion of the glass fragments to the film, to preventglass splinters from being sprayed and consequently preventing injury topeople.

[0004] Moreover, it is becoming increasingly desirable, for bettercomfort, for the insert to allow the glazing also to meet acousticperformance criteria so as to attenuate the perception of airborneand/or solid-borne noise reaching the passenger compartment via theglazing.

[0005] Polyvinyl butyral (PVB) is widely used for its mechanicalproperties. It may also provide the laminated glazing with acousticproperties when its composition, including in particular its content ofplasticizers, is very suitable.

[0006] The insert is selected for providing acoustic properties using amethod of determining the critical frequency of the laminated glass andits comparison with the critical frequency of a glass bar. Such a methodis described in patent EP-B-0 100 701; an insert is regarded as beingsuitable when a bar 9 cm in length and 3 cm in width made of a laminatedglass, which comprises two glass sheets 4 mm in thickness joinedtogether by said insert 2 mm in thickness, has a critical frequency thatdiffers by at most 35% from that of a glass bar having the same lengthand the same width and a thickness of 4 mm.

[0007] This method of selection, valid for any type of insert intendedfor being incorporated into laminated glazing, is not only applicable toPVB but also to other polymeric films.

[0008] Now, whether used in PVB-laminated glazing or glazing laminatedwith other polymeric films, in combination or otherwise, so as to obtain“acoustic” glazing, it is paramount for the insert to meet mechanicalstrength criteria.

[0009] This is because building or automobile glazing is required toexhibit excellent response capabilities in terms of impact protection,such as accidental collisions, the falling of an object or a person,vandalism and break-in by throwing objects. Most glazing in use must atleast meet the criteria of European Standard EN 356 up to Class P2A.

[0010] One method of assessing the tear strength of the insert is knownfrom European patent application EP 1 151 855. For a given thickness ofthe insert, the value of the critical energy J_(c) of the insert, whichis representative of the energy needed to propagate a crack initiated inthe insert, is calculated and if this value is greater than a referencevalue the insert meets the tear strength criterion.

[0011] However, the inventors have demonstrated that some insertsalthough meeting the tear strength criterion are, nevertheless, notcompletely satisfactory from the mechanical strength standpoint.

[0012] Consequently, the object of the invention is to provide a methodof selecting the quality of the mechanical strength of an insert,optionally one that also exhibits acoustic insulation properties, whichcompletes the existing one described in patent application EP 1 151 855so as to completely guarantee the effectiveness of the insert used forimpact protection.

[0013] According to the invention, the method, which consists inevaluating the tear strength of the insert, is characterized in that italso consists in evaluating the adhesion of said insert to at least oneglass substrate.

[0014] According to one feature, the tear strength is evaluated by:

[0015] determining the value of the critical energy J_(c) of the insert,this value being representative of the energy needed to propagate acrack initiated in the insert;

[0016] calculating the value of the critical energy normalized tothickness {tilde over (J)}_(c) and defined by the equation {tilde over(J)}_(c)=J_(c)×e₁, e₁ being the thickness of the insert; and

[0017] comparing {tilde over (J)}_(c) with a reference value {tilde over(J)}_(ref) representative of a reference insert formed by a PVB film0.38 mm in thickness, and equal to 13.3 J/m, the insert meeting the tearstrength criterion when {tilde over (J)}_(c)>{tilde over (J)}_(ref).

[0018] According to another feature, the adhesion of the insert isevaluated by twisting a specimen of the insert fastened to two glasssubstrates, by measuring the value of the torsional force or torque forwhich the separation of the insert from at least one of the substratesis initiated, by calculating the shear strength τ from this value and bycomparing this value with a warning value established for a referenceinsert formed from PVB.

[0019] According to another feature, the mechanical strength of thereference insert with respect to its thickness is mathematically in theform of an approximately parabolic function defined by the criticalenergy J_(c) as a function of the adhesive strength τ. The insert whosemechanical strength is to be evaluated meets the tear and adhesivestrength criteria when, after evaluating the critical energy andadhesive strength values, these values lie within the parabolic curvethat has a minimum corresponding to a value of the critical energy J_(c)equal to 17,500 J/m².

[0020] The insert is selected when, at a temperature of 20° C., itscritical energy value is greater than 17,500 J/m² and its shear strengthτ is between 3.8 and 6.9 MPa.

[0021] In particular, the insert is selected when, at a temperature of20° C., its critical energy value is greater than 22,500 J/m² and itsshear strength τ is between 4.8 and 6.1 MPa.

[0022] The method according to the invention also consists in selectingthe insert for the acoustic properties given to the laminated glazing,the insert meeting the selection of acoustic insulation properties isespecially chosen when a bar 9 cm in length and 3 cm in width made of alaminated glass, which comprises two glass sheets 4 mm in thicknessjoined together by said insert 2 mm in thickness, has a criticalfrequency which differs by at most 35% from that of a glass bar havingthe same length and the same width and a thickness of 4 mm.

[0023] Preferably, the device which evaluates the shear strength of aninsert is characterized according to the invention in that it comprisestwo jaw systems intended to sandwich a glazing specimen consisting oftwo glass substrates and the insert, one of the systems being fixedwhile the other is capable of being moved and rotated, a shaft forrotating the movable-jaw system, a motor for rotating the shaft, atorque meter placed between the motor and the movable-jaw system, and abox that houses the computing elements and includes a display part onwhich the value of the strength may be read.

[0024] The invention also relates to a polymeric insert intended to beincorporated in laminated glazing, characterized in that it has, at atemperature of 20° C., a critical energy value of greater than 17,500J/m², preferably greater than 22,500 J/m², and a shear strength τ ofbetween 3.8 and 6.9 MPa, preferably between 4.8 and 6.1 MPa.

[0025] According to one feature, the insert has a thickness of at least0.76 mm.

[0026] According to another feature, the insert has a thickness e equalto at least ${e_{ref} \times \frac{J_{ref}}{J_{c}}},$

[0027] where:

[0028] J_(c) is the critical energy value specific to the material ofthe insert and representative of the energy needed to propagate a crackinitiated in the insert;

[0029] J_(ref) is a reference critical energy value corresponding to thecritical energy value of a polyvinyl butyral (PVB) film and equal to35,100 J/m² for a temperature of 20° C. and for a stretch rate on thePVB film of 100 mm/min; and

[0030] e_(ref) is a reference thickness corresponding to that of the PVBfilm and equal to 0.38 mm.

[0031] Advantageously, the insert gives acoustic insulation propertiesto the laminated glazing for which it is intended. In particular, it issuch that a bar 9 cm in length and 3 cm in width, made of a laminatedglass comprising two glass sheets 4 mm in thickness joined together bysaid insert 2 mm in thickness, has a critical frequency which differs byat most 35% from that of a glass bar having the same length and the samewidth and a thickness of 4 mm.

[0032] The insert comprises one or more polymeric elements, preferablyat least one PVB film.

[0033] Finally, the invention relates to laminated glazing comprising atleast two glass sheets and at least one polymeric insert, especially aPVB-based insert, characterized in that the insert has, at a temperatureof 20° C., a critical energy value of greater than 17,500 J/m²,preferably greater than 22,500 J/m², and a shear strength τ of between3.8 and 6.9 MPa, preferably between 4.8 and 6.1 MPa.

[0034] Advantageously, this glazing is glazing for a vehicle, comprisingtwo glass sheets each having a thickness of between 1.2 and 2.5 mm, andan insert joined to the two glass sheets and having a thickness of atleast 0.76 mm.

[0035] Preferably, the insert gives this glazing acoustic insulationproperties, that is to say, in particular, the insert is such that a bar9 cm in length and 3 cm in width, made of a laminated glass comprisingtwo glass sheets 4 mm in thickness joined together by said insert 2 mmin thickness, has a critical frequency which differs by at most 35% fromthat of a glass bar having the same length and the same width and athickness of 4 mm.

[0036] Other advantages and features of the invention will becomeapparent on reading the description which follows, in conjunction withthe appended drawings in which:

[0037]FIG. 1 is a sectional view of single laminated glazing having onlyone insert film;

[0038]FIG. 2 illustrates diagrammatically an experimental device forevaluating the tear strength of the insert;

[0039]FIG. 3 shows the variation in the energy of the crack root, whichcrack is produced in the insert;

[0040]FIG. 4 shows the tensile force exerted on the insert as a functionof the stretch of this insert;

[0041]FIG. 5 shows the potential energy of the insert as a function ofthe stretch of this insert;

[0042]FIG. 6 illustrates a schematic front view of an experimentaldevice for evaluating the adhesion of the insert to the substrate towhich it is joined;

[0043]FIG. 7 illustrates a sectional side view of the device shown inFIG. 6;

[0044]FIG. 8 illustrates the curve of the critical energy as a functionof the shear strength for PVB 0.76 mm in thickness; and

[0045]FIG. 9 illustrates a profile view of one embodiment of the devicefor evaluating the adhesion of the insert to the substrate to which itis joined.

[0046] The method of the invention is intended for selecting an insertas regards its mechanical strength, the insert being intended to beincorporated into a single or multiple laminated glazing unit, whichmust withstand hard impacts (EN 356 standard up to Class P2A) or softimpacts (EN 12600 standard). The purpose of the method is to selectwithout having to evaluate mechanical strength by a destructive impacteffect.

[0047] In the example below, it is desired to know whether an insert issuitable for it to be incorporated into laminated glazing, such asbuilding or automobile glazing.

[0048] The single laminated glazing shown in FIG. 1 comprises two glasssubstrates 10 and 11 between which is fastened an insert 12.

[0049] To select the insert, it is therefore necessary to evaluate itsmechanical strength. The inventors have demonstrated that two parametersshould be evaluated, namely the tear strength of the insert and theadhesion of the insert to the substrate to which it is joined.

[0050] The tear strength is evaluated on the basis of the test andcalculation method explained in patent application EP 1 151 855, whichwe repeat here.

[0051] The tear strength of the insert depends on the type of materialof which it is made and on its thickness. It is characterized by anenergy value representative of the energy needed to propagate a crackinitiated in the material. This energy, called the critical energyJ_(c), is different for each type of material and is independent of thethickness of the film, it being expressed in J/m².

[0052] The tear strength or critical energy J_(c) is given in a knownmanner by an energy method based on the Rice integral J, which definesthe energy localized at the root of a crack in a film subjected to veryhigh stresses at the location of a crack. It may be written in thesimplified mathematical form (1):${J = {{- \frac{1}{e_{1}}}\left( \frac{\partial U}{\partial a} \right)}},$

[0053] for a given stretch δ of the specimen tested which will hereafterbe called the displacement δ, and in which:

[0054] e₁ is the thickness of the specimen;

[0055] a is the length of the crack; and

[0056] U is the potential energy of the specimen.

[0057] The above method for calculating the crack root energy J is thatdeveloped by Tielking.

[0058] The experimental device as illustrated in FIG. 2 is thefollowing:

[0059] Tensile tests using a tension-compression machine 2 are carriedout on several specimens, for example four specimens Ex₁ to Ex₄, of thesame material and with the same surface area of 100 mm² (50 mm in lengthby 20 mm in width). Each specimen is notched according to the reference20 on its sides and perpendicularly to the tensile force, with adifferent crack length a for each specimen Ex₁ to Ex₄, corresponding to5, 8, 12 and 15 mm respectively.

[0060] Each specimen Ex is stretched perpendicularly to the cracks 20 ata stretch rate of 100 mm/min over a given stretch length or distance δand in an environment in which the temperature is 20° C.

[0061] This method is used to establish a curve of variation C (FIG. 3)of the crack root energy J as a function of the displacement δ undergoneby the specimen and to determine, from this curve, the critical energyJ_(c) for initiating tearing in the specimen.

[0062] It is therefore at this critical value J_(c) that the materialtears and that it is consequently mechanically damaged as regards therequired mechanical function.

[0063] Curve C is obtained from the steps that we explain below. Thespecimens here are polyvinyl butyral films having a thickness of 0.38mm.

[0064] Firstly, for each of these specimens Ex₁ to Ex₄, curve C1 (FIG.4) representative of the tensile force exerted on the specimen isplotted as a function of the displacement δ undergone by said specimen,which displacement goes from 0 to 40 mm.

[0065] From the curves C1 of the specimens, the potential energy Ucorresponding to a displacement δ given as a function of the increasedlength a of the crack relative to its initial length is then deduced.The potential energy U is measured by calculating the area A, equivalentto the hatched area shown in FIG. 4, under the curve C1 between 0 mm andthe given displacement δ, here 22 mm in the case of the hatched area andcorresponding to specimen Ex₄.

[0066] Eight displacements δ from 3 mm to 22 mm were considered. It isthen possible to plot, for each of the eight displacements, a curve C2illustrated in FIG. 5, representing the potential energy U as a functionof the length a to which the crack has grown.

[0067] Curve C2 representative of the potential energy U is a straightline; consequently, the derivative (∂U/∂a), formulated in equation (1),of the energy J is in fact the slope of the line C2 and therefore equalto a constant. The value of J is calculated by dividing this constant bythe thickness e₁ of the specimen.

[0068] After calculating each of the slopes corresponding to the eightdisplacements, curve C (FIG. 3) representative of the energy J as afunction of the displacement δ is established.

[0069] A video camera, which displays the propagation of the crack 20,is used to detect at which displacement δ_(c) propagation of the crackin the specimen starts. Using curve C, the corresponding value of thecritical energy J_(c) is deduced from this displacement δ_(c).

[0070] This critical value J_(c) of 35,100 J/m² in the case of PVBconstitutes the reference value J_(ref) of the energy, above which anyenergy value calculated for another material and according to the methodexplained above will be considered to be correct so that this materialis suitable for meeting the mechanical strength criteria.

[0071] Once the specific critical energy value J_(c) has beencalculated, it is, as already explained above, normalized with respectto its thickness, {tilde over (J)}_(c) (J_(c)×e₁) so as to be comparedwith the reference value of PVB equal to 13.3 J/m and so as to deducetherefrom the suitable thickness e when the thickness e₁ isinsufficient.

[0072] As regards the adhesion of the insert to the substrate to whichit is joined, this is evaluated in the following manner.

[0073] The adhesion test consists in applying a torsional force to aspecimen of the laminated glazing until separation of the insert from atleast one of the substrates is initiated.

[0074] The test is carried out on a round specimen 30 of radius r equalto 10 mm by means of a torsion device 3 of known type illustrated inFIG. 6.

[0075] The device comprises three jaws 31, 32, 33, a pulley 34 of radiusR equal to 100 mm and connected to a drive chain 35 of vertical axis.The jaws are in the form of circular arcs each of 120°, so as to clampthe entire specimen. The surface coating of the jaws is made of amaterial mechanically compatible with glass, for example aluminum,Teflon® or polyethylene.

[0076] One of the jaws is kept fixed against a frame 36 (FIG. 7), whileanother jaw is fixed to the pulley 34 which is intended to rotate inorder to exert torsion on the specimen.

[0077] Rotation of the pulley is caused by the displacement of the chain34 connected to the pulley. The chain is pulled at a constant speed ofat least 35 to 50 mm/min.

[0078] The force F needed for the onset of debonding of the insert toappear when the specimen is twisted is measured using a force sensor.

[0079] The shear strength-may be calculated therefrom by the knownformula: $\tau = \frac{2{FR}}{\pi \quad r^{3}}$

[0080] in which it will be recalled that F is the force needed for theonset of debonding of the insert to occur, R is the radius of the pulleyand r is the radius of the specimen.

[0081] However, this device is bulky and the tests must therefore becarried out in the laboratory. It is thus ill-suited to measurements ofthe “process indicator” type on a laminated glazing manufacturing line.

[0082] Now, for the manufacture of laminated glazing, although thecomposition of the polymeric insert is designed to meet the strengthvalues set by the invention, poor adhesion of the insert maynevertheless occur in the finished product because of parametersassociated with the process for manufacturing the glazing.

[0083] These may, for example, be the conditions under which the insertis stored; if the moisture content is not adequate, the PVB hydroxylbonds may be altered by water, which will impair the bonding of theinsert to the glass. Poor adhesion may also be due to poor washing ofthe glass and the deposition of ions may result in hydroxyl consumption.The calendering step during assembly of the glass and the insert alsoacts on the quality of the bonding, the temperature and the compressionforces having to be properly controlled.

[0084] Thus, the inventors have developed a measurement device otherthan that described above which is advantageously more compact andeasily transportable so as to carry out measurements during monitoringof the manufacture close to the manufacturing line so that it becomespossible to quickly intervene in the process in response to poormeasured strength values. This device thus constitutes a management toolfor assessing the quality of manufacture of laminated glazing.

[0085] Miniaturized to about 60 cm by 20 cm, the device 4 shown in FIG.9 comprises two three-jaw systems 40 and 41, a rotation shaft 42, amotor 43 for driving the shaft, a torque meter 44 and a box 45 housingthe computing elements.

[0086] The round laminated glazing specimen is intended to be sandwichedbetween the two jaw systems 40 and 41, one of the systems 40 being fixedwhile the other is capable of being moved and rotated by means of itsconnection to the shaft 42. The torque meter is placed between the motorand the movable-jaw system 41. The rotation speed of the shaft dependson the thickness of the film. To take an example, for a film 0.76 mm inthickness, the rotation is around 0.08 revolutions per minute.

[0087] The system 41 rotates and when the measured torque reverses, theinitiation of debonding of the insert has taken place. The torque meteris connected to the computing elements of the box 45, which includes adisplay part on which the value of the strength τ may be read directly.The adhesion is suitable if this value lies within the range defined bythe invention, as explained below.

[0088] To have a detailed understanding of the scatter in the value ofthe strength τ, it is preferred to repeat the test on several specimens,for example, a minimum number of 5 specimens, and to calculate anaverage of the strength together with its standard deviation.

[0089] Finally, the strength value is compared with a warning rangewithin which any value is appropriate for meeting the adhesioncriterion. The warning range of the adhesive strength τ is equal to3.8-6.9 MPa. This warning range was established from a PVB film which,it will be recalled, is considered at the present time as an inserthaving the highest performance with regard to mechanical strength formeeting the EN356 standard, in particular the characterizing Class P2Aof a PVB 0.76 mm in thickness.

[0090] To make it easier to compare any insert with the referenceinsert, which is PVB, the inventors have demonstrated that themechanical strength may be defined by a reference curve representativeof the critical energy J_(c) as a function of the adhesive strength,this curve having an approximately parabolic shape.

[0091] To take an example, FIG. 7 shows this curve for a PVB insertthickness of 0.76 mm. Since the critical energy varies according to thethickness, for a 0.76 mm thickness the reference value of the energy is17,500 J/m². Since in this graph the energy is not normalized withrespect to thickness, it is necessary for any comparison with this curveto test an insert having the same thickness.

[0092] Consequently, the minimum value that a critical energy must meetcorresponds to the minimum of the curve, namely 17,500 J/m², and theadhesive strength must lie within a range centered on the value 5.5 MPaand broadening with the increasing critical energy value. Thus, thecritical energy and the adhesive strength values measured on an insertto be tested which lie inside this parabolic curve mean that the inserttested is regarded as being satisfactory from the mechanical strengthstandpoint.

[0093] In order to meet the two criteria—tear strength and adhesivestrength—with acceptable reproducibility, the insert must have acritical energy J_(c) of greater than 17,500 J/m² and an adhesivestrength of between 3.8 and 6.9 MPa. Below 3.8 and above 6.9 MPa in thecase of the adhesive strength, the laminated glass assembly has too higha probability of poor mechanical behavior. Between 3.8 and 4.8 andbetween 6.7 and 6.9 MPa, the insert may be regarded as meeting theexpected mechanical strength, while not being optimum.

[0094] For an optimum insert, a region necessarily lying within theparabola, here the region B, will preferably be considered. In addition,an insert having a minimum thickness of 0.76 mm meeting the Class P2Arequirements will be chosen when, preferably, the critical energy J_(c)is greater than 22,500 J/m² and the adhesive strength τ is between 4.8and 6.1 MPa.

[0095] Other ranges of the adhesive strength τ may be demonstrated foreach type of impact, especially on having a relatively low impact energyand with an extensive contact area (soft impact).

[0096] If it is desired to select an insert for laminated glazing havingboth acoustic insulation properties and mechanical strength, the insertwill be firstly chosen for its acoustic performance. To achieve this,the reader may refer to the patent EP-B-0 100 701 or patent applicationEP 0 844 075 which indicate two selection technique variants, these alsobeing summarized in the abovementioned Patent Application EP 1 151 855.

[0097] In particular, an insert has acoustic insulation properties whena bar 9 cm in length and 3 cm in width, made of laminated glasscomprising two glass sheets 4 mm in thickness joined together by saidinsert 2 mm in thickness, has a critical frequency which differs by atmost 35% from that of a glass bar having the same length and the samewidth and a thickness of 4 mm.

[0098] Once the material has been chosen, its adhesion is evaluated bycalculating its shear strength which, if this lies within the desiredrange, for example 4.8-6.1 MPa for meeting the EN356 standard accordingto Class P2A, therefore meets the adhesion criterion. Finally, itsthickness for meeting the mechanical strength criterion is determined.The thickness e of the insert must at least be equal to${e_{ref} \times \frac{J_{ref}}{J_{c}}},$

[0099] where:

[0100] J_(c) is the critical energy value specific to the material ofthe insert and representative of the energy needed to propagate a crackinitiated in the insert;

[0101] J_(ref) is a reference critical energy value which corresponds tothe critical energy value of a polyvinyl butyral (PVB) film and equal to35,100 J/m² for a temperature of 20° C. and for a stretch rate on thePVB film of 100 mm/min; and

[0102] e_(ref) is a reference thickness corresponding to that of the PVBfilm and equal to 0.38 mm.

[0103] The tear strength of the material, which will therefore beidentified directly with the critical energy J_(c), is evaluated onlyafter assessing the acoustic performance of said material and itsadhesion. This is because, for the purpose of using an insert forlaminated glazing providing acoustic insulation and having to meet theimpact protection standards, the approach adopted by the invention is tofirstly choose the material suitable for meeting the acoustic insulationcriteria and then to test the adhesion performance of this material soas to deduce therefrom the thickness e needed to meet the tear strengthcriteria.

[0104] It should be noted that satisfactory glazing from the mechanicalstrength standpoint may comprise a monolithic insert of thickness e or aplurality of inserts separated by various substrates, the sum of thethicknesses of the inserts corresponding to the calculated thickness e.

1. A method for selecting a polymeric insert intended to be chosen forits mechanical strength qualities so as to be incorporated into theconstruction of laminated glazing, the method consisting in evaluatingthe tear strength of the insert, characterized in that it also consistsin evaluating the adhesion of said insert to at least one glasssubstrate.
 2. The method as claimed in claim 1, characterized in thatthe tear strength is evaluated by: determining the value of the criticalenergy J_(c) of the insert, this value being representative of theenergy needed to propagate a crack initiated in the insert; calculatingthe value of the critical energy normalized to thickness {tilde over(J)}_(c)and defined by the equation {tilde over (J)}_(c)=J_(c)×e₁, e1being the thickness of the insert; and comparing {tilde over (J)}_(c)with a reference value {tilde over (J)}_(ref) representative of areference insert formed by a PVB film 0.38 mm in thickness, and equal to13.3 J/m, the insert meeting the tear strength criterion when {tildeover (J)}_(c)>{tilde over (J)}_(ref).
 3. The method as claimed in claim1 or 2, characterized in that the adhesion of the insert is evaluated bytwisting a specimen of the insert fastened to two glass substrates, bymeasuring the value of the torsional force or torque for which theseparation of the insert from at least one of the substrates isinitiated, by calculating the shear strength τ from this value and bycomparing this value with a warning value established for a referenceinsert formed from PVB.
 4. The method as claimed in claim 1,characterized in that the mechanical strength of the reference insertwith respect to its thickness is mathematically in the form of anapproximately parabolic function defined by the critical energy J_(c) asa function of the adhesive strength τ.
 5. The method as claimed in claim4, characterized in that the insert whose mechanical strength is to beevaluated meets the tear and adhesive strength criteria when, afterevaluating the critical energy and adhesive strength values, thesevalues lie within the parabolic curve that has a minimum correspondingto a value of the critical energy of 17,500 J/m².
 6. The method asclaimed in any one of the preceding claims, characterized in that theinsert is selected when, at a temperature of 20° C., its critical energyvalue is greater than 17,500 J/m² and its shear strength τ is between3.8 and 6.9 MPa.
 7. The method as claimed in claim 6, characterized inthat the insert is selected when, at a temperature of 20° C., itscritical energy value is greater than 22,500 J/m² and its shear strengthτ is between 4.8 and 6.1 MPa.
 8. The method as claimed in any one of thepreceding claims, characterized in that it also consists in selectingthe insert for the acoustic properties given to the laminated glazing,the insert meeting the selection of acoustic insulation properties isespecially chosen when a bar 9 cm in length and 3 cm in width made of alaminated glass, which comprises two glass sheets 4 mm in thicknessjoined together by said insert 2 mm in thickness, has a criticalfrequency that differs by at most 35% from that of a glass bar havingthe same length and the same width and a thickness of 4 mm.
 9. A devicefor implementing the method as claimed in any one of the precedingclaims, characterized in that the device evaluates the shear strength ofan insert and comprises two jaw systems intended to sandwich a glazingspecimen consisting of two glass substrates and the insert, one of thesystems being fixed while the other is capable of being moved androtated, a shaft for rotating the movable-jaw system, a motor forrotating the shaft, a torque meter placed between the motor and themovable-jaw system, and a box that houses the computing elements andincludes a display part on which the value of the strength may be read.10. A polymeric insert intended to be incorporated in laminated glazing,characterized in that it has, at a temperature of 20° C., a criticalenergy value of greater than 17,500 J/m², preferably greater than 22,500J/m², and a shear strength τ of between 3.8 and 6.9 MPa, preferablybetween 4.8 and 6.1 MPa.
 11. The insert as claimed in claim 10,characterized in that it has a thickness of at least 0.76 mm.
 12. Theinsert as claimed in claim 10 or 11, characterized in that it has athickness e equal to at least ${e_{ref} \times \frac{J_{ref}}{J_{c}}},$

where: J_(c) is the critical energy value specific to the material ofthe insert and representative of the energy needed to propagate a crackinitiated in the insert; J_(ref) is a reference critical energy valuecorresponding to the critical energy value of a polyvinyl butyral (PVB)film and equal to 35,100 J/m² for a temperature of 20° C. and for astretch rate on the PVB film of 100 mm/min; and e_(ref) is a referencethickness corresponding to that of the PVB film and equal to 0.38 mm.13. The insert as claimed in one of claims 10 to 12, characterized inthat it gives acoustic insulation properties to the laminated glazingfor which it is intended.
 14. The insert as claimed in claim 13,characterized in that it is such that a bar 9 cm in length and 3 cm inwidth, made of a laminated glass comprising two glass sheets 4 mm inthickness joined together by said insert 2 mm in thickness, has acritical frequency which differs by at most 35% from that of a glass barhaving the same length and the same width and a thickness of 4 mm. 15.The insert as claimed in one of claims 10 to 13, characterized in thatit comprises one or more polymeric elements.
 16. The insert as claimedin claim 15, characterized in that it comprises at least one PVB film.17. Laminated glazing comprising at least two glass sheets and at leastone polymeric insert, especially a PVB-based insert, characterized inthat the insert has, at a temperature of 20° C., a critical energy valueof greater than 17,500 J/m², preferably greater than 22,500 J/m², and ashear strength τ of between 3.8 and 6.9 MPa, preferably between 4.8 and6.1 MPa.
 18. The laminated glazing as claimed in claim 17, characterizedin that the glazing is glazing for a vehicle, comprising two glasssheets each having a thickness of between 1.2 and 2.5 mm, and an insertjoined to the two glass sheets and having a thickness of at least 0.76mm.
 19. The laminated glazing as claimed in claim 17 or 18,characterized in that the insert gives it acoustic insulationproperties.
 20. The laminated glazing as claimed in claim 19,characterized in that the insert is such that a bar 9 cm in length and 3cm in width, made of a laminated glass comprising two glass sheets 4 mmin thickness joined together by said insert 2 mm in thickness, has acritical frequency which differs by at most 35% from that of a glass barhaving the same length and the same width and a thickness of 4 mm.